With constantly heated sealing bars for providing sealing seams on film webs passed in a timed sequence between the bars, there occurs the problem that, during a temporary stoppage of the apparatus during which the sealing bars lifted off from the film web are kept at the operating temperature, the heat radiated by the bars melts the film web situated between the bars and which is not moved. To prevent such melting during operational stoppages, for example, during a lunch break, it is known from practice that heat-screening protective plates are moved in between the run out sealing bars and the film web situated between them which is to be provided with sealing seams. However, because of the narrow space available for installation, the arrangement of such protective plates, requires a high expenditure on design, which leads to a considerable increase in the cost of the apparatus, in particular when several sealing apparatuses are arranged one after the other, whereof those first in line preheat the film to be sealed and the last effects the final seal.